The present invention relates to reagents and methods for rendering a surface biocompatible, and in particular to reagents and methods for xe2x80x9cpassivatingxe2x80x9d the surface of an implantable medical device in order to render it hemocompatible. In another aspect, the invention relates to biomedical devices, per se, and in particular those having biocompatible, including hemocompatible, tissue-contacting surfaces.
Manufacturers of implantable medical devices have long attempted to understand, and in turn improve, the performance of materials used in blood-contacting applications (Leonard, E. F., et al. Ann. N.Y. Acad. Sci. 516, New York, Acad. Sci., New York, 1987). The biological response of the body, as well as problems with infection, have hindered the application of implantable, disposable, and extracorporeal devices. Anticoagulant drugs, such as heparin and coumadin, can improve the use of such devices, although anticoagulants have their own corresponding risks and drawbacks. For these reasons, development of materials having greater compatibility with blood has been pursued aggressively (Sevastianov, V. I., CRC Crit. Rev. Biocomp. 4:109, 1988).
Two general strategies that have been used to develop improved blood-contacting materials include modifying the chemistry of the bulk material itself, and/or modifying the interfacial properties of the material. With regard to the latter approach, several classes of materials have been covalently bonded onto blood-contacting surfaces with the goal of improving blood compatibility. These include anticoagulants, such as heparin and hirudin; hydrogels; polyethylene oxide (PEO); albumin binding agents; cell membrane components; prostaglandins; and sulfonated polymers. These approaches have met with varying degrees of success in terms of reducing protein adsorption, platelet adhesion and activation, and thrombus formation. Unfortunately, no approach has yet been shown to be universally applicable for improving blood-biomaterial interactions.
As mentioned above, albumin binding agents have been considered for use on biomaterials. Biomaterials having a high surface concentration of albumin have been shown to be less likely to initiate the fibrin cascade and platelet attachment than those having a high concentration of other serum proteins, such as fibrinogen, fibronectin, or immunoglobulins. On many polymeric materials, however, fibrinogen is often the predominant protein adsorbed from protein mixtures or plasma. For these reasons, investigators have attempted to immobilize albumin onto materials or to design biomaterial surfaces that will enhance binding of endogenous albumin from blood, thus mitigating the adsorption of fibrinogen and consequent thrombogenic phenomena.
In this respect, a number of different approaches have been employed to date. These approaches include passive adsorption or covalent immobilization of albumin to the surface, and the development of surfaces designed to selectively bind endogenous albumin from circulating blood, the latter using alkyl chain-modified materials and other means.
Munro, et al., U.S. Pat. No. 4,530,974, discloses a method of adsorbing albumin to a water-insoluble polymer such as polyurethane by covalently binding to the surface a nonionic hydrophobic aliphatic chain to which serum albumin will selectively bind.
Frautschi et al., U.S. Pat. Nos. 5,017,670 and 5,098,977, teach methods for covalent attachment of aliphatic extensions of 12 to 22 carbon atoms to water-insoluble polymers containing aromatic rings and ring structures with adjacent secondary hydroxyls for increased albumin binding.
Eaton, U.S. Pat. No. 5,073,171, describes a biocompatible prosthetic device incorporating an amount of an albumin binding dye effective to form a coating of endogeneous albumin on the device when the device is in contact with a physiological fluid containing albumin.
While some or all of these various strategies can be used to enhance the binding of endogenous albumin to blood-contacting material surfaces, and in turn to reduce fibrinogen binding, these approaches are each limited in one or more respects. Alkyl chain-modified surfaces have been shown to increase albumin binding and decrease fibrinogen binding, but these effects were fairly limited, for instance, to a short term time frame (generally less than one hour). In addition, various other surface modification methods discussed above are useful for only a narrow range of substrate materials.
On another subject, the assignee of this application has developed the ability to attach bioactive groups to a surface by covalently bonding those groups, directly or indirectly, to the surface. For instance, U.S. Pat. Nos. 4,722,906, 4,979,959, 4,973,493 and 5,263,992 relate to devices having biocompatible agents covalently bound via photoreactive groups and a chemical linking moiety to the biomaterial surface. U.S. Pat. Nos. 5,258,041 and 5,217,492 relate to the attachment of biomolecules to a surface through the use of long chain chemical spacers. U.S. Pat. Nos. 5,002,582 and 5,512,329 relate to the preparation and use of polymeric surfaces, wherein polymeric agents providing desirable properties are covalently bound via a photoreactive moiety to the surface. In particular, the polymers themselves exhibit the desired characteristics, and in the preferred embodiment, are substantially free of other (e.g., bioactive) groups.
It would be highly desirable to be able to attach albumin to a biomaterial surface in a manner that is suitably stable for extended use, particularly in a manner that permits the albumin to be replenished over time and in the course of use.
The present invention provides a novel reagent for use in passivating a biomaterial surface, the reagent comprising a latent reactive group and a bifunctional aliphatic acid, in combination with a spacer group linking the latent reactive group to the aliphatic acid in a manner that preserves the desired function of each group. The reagent can be used to passivate a surface by activating the latent reactive group in the presence of the surface in order to covalently bond the reagent to the surface. Once bound to the surface, the reagent presents the aliphatic acid to the physiological environment, in vivo, in a manner (e.g., concentration and orientation) sufficient to hold and orient albumin. Preferably, over time, the reagent surface is able to replenish itself by replacing albumin molecules that have become unbound or deteriorated with new albumin molecules. Albumin (e.g., human serum albumin (HSA)), is defined as any naturally occurring proteinaceous moiety containing a fatty acid binding site.
In a preferred embodiment, the reagent is of the general formula (X)mxe2x80x94Yxe2x80x94(Z)n where X is a latent reactive (e.g., photoreactive) group, Y is a spacer radical, and Z is a bifunctional aliphatic acid, as each are described herein. The values of m and n are xe2x89xa71 and while m can equal n, it is not necessary. The aliphatic acid is xe2x80x98bifunctionalxe2x80x99 in that it provides both an aliphatic region and an anionic (e.g., carboxylic acid) region. Once attached to a surface, these portions cooperate in the process of attracting and binding of albumin in order to passivate the surface.
In the preferred embodiment where both m and n=1, the reagent is termed a heterobifunctional reagent. The aliphatic acid is preferably attached to the latent reactive group by means of a divalent spacer group in a manner that does not detrimentally affect the function of either the aliphatic or anionic portions. Higher-valent spacer groups can also be selected which permit the attachment of multiple aliphatic acid and latent reactive groups, again in a manner which does not detrimentally affect the functions of the respective groups. In this case m does not necessarily equal n and both are xe2x89xa71.
In a further embodiment, the spacer group can be a multivalent polymer having multiple sites along the backbone which permit covalent attachment of the aliphatic acid and latent reactive groups. These groups can be attached to a preformed reactive polymer using conventional chemical coupling techniques or may be incorporated during the polymerization process by use of appropriately substituted monomers. In this embodiment, m does not necessarily equal n and typically both are larger than one.
The invention further provides a method for preparing a passivating reagent, as well as a method of using the reagent to passivate the surface of a synthetic or natural biomaterial. In yet a further embodiment, the invention provides a surface coated with a passivating reagent of this invention, and in turn, an article fabricated from a material providing a surface coated or coatable with such a reagent. In yet a further embodiment, the invention provides a passivated biomaterial surface having reagent attached thereto and albumin attracted and attached to the bound reagent.
The present invention permits the binding of albumin to a surface to be enhanced by the use of a surface modification reagent. The reagent includes a bifunctional aliphatic acid capable of being attached to a surface in an amount and orientation that improves the ability of the surface to attract and bind albumin. While not intending to be bound by theory, it appears that a surface bearing a reagent of this invention exhibits improved albumin binding by virtue of both hydrophobic interactions (of the alkyl chain) and ionic interactions (of the anionic moiety) with albumin. It is expected that the hydrophobic interactions serve to hold and orient the free albumin molecule, while the ionic interactions serve to maintain the albumin molecule in position by the addition of attractive ionic forces. In a particularly preferred embodiment, the bifunctional aliphatic acid is attached to either alkane, oxyalkane, or hydrophobic polymeric backbones to allow both aliphatic and ionic regions of the bifunctional acid analog to spacially orient away from the biomaterial surface to induce better binding with native albumin. The reagent, in turn, permits albumin binding surfaces to be created using a variety of medical device materials, and in particular, for use in blood-contacting medical devices.
Bifunctional Aliphatic Acid
The bifunctional aliphatic acid of the present invention (xe2x80x9cZxe2x80x9d group) includes both an aliphatic portion and an anionic portion. The word xe2x80x9caliphaticxe2x80x9d, as used herein, refers to a substantially linear portion, e.g., a hydrocarbon backbone, capable of forming hydrophobic interactions with albumin. The word xe2x80x9canionicxe2x80x9d, in turn, refers to a charged portion capable of forming further ionic interactions with the albumin molecule. By the use of a reagent of this invention, these portions can be covalently attached to a surface in a manner that retains their desired function, in order to attract and bind native albumin from blood and other bodily fluids.
In a preferred embodiment, the invention includes photoactivatible molecules having fatty acid functional groups, including polymers having multiple photoactivatible and fatty acid functional groups, as well as heterobifunctional molecules. Photoactivatible polyacrylamide copolymers containing multiple pendant fatty acid analogs and multiple pendant photogroups have been synthesized from acrylamide, a benzophenone-substituted acrylamide, and N-substituted acrylamide monomers containing the fatty acid analog. Photoactivatible polyvinylpyrrolidones have also been prepared in a similar fashion. Polyacrylamide or polyvinylpyrrolidone copolymers with a single end-point photogroup and multiple pendant fatty acid analogs have also been synthesized. Finally, photoactivatible, heterobifunctional molecules having a benzophenone on one end and a fatty acid group on the other end optionally separated by a spacer have been made, wherein that spacer can be a hydrophobic alkyl chain or a more hydrophilic polyethyleneglycol (PEG) chain.
Spacer Group
Suitable spacers (xe2x80x9cYxe2x80x9d groups) for use in preparing heterobifunctional reagents of the present invention include any di- or higher-functional spacers capable of covalently attaching a latent reactive group to an aliphatic acid in a manner that permits them both to be used for their intended purpose. Although the spacer may itself provide a desired chemical and/or physical function, preferably the spacer is non-interfering, in that it does not detrimentally affect the use of the aliphatic and ionic portions for their intended purposes. In the case of the polymeric reagents of the invention, the spacer group serves to attach the aliphatic acid to the backbone of the polymer.
The spacer may be either aliphatic or polymeric and contain various heteroatoms such as O, N, and S in place of carbon. Constituent atoms of the spacers need not be aligned linearly. For example, aromatic rings, which lack abstractable hydrogen atoms (as defined below), can be included as part of the spacer design in those reagents where the latent reactive group functions by initiating covalent bond formation via hydrogen atom abstraction. In its precursor form (i.e., prior to attachment of a photoreactive group and aliphatic acid), a spacer can be terminated with any suitable functionalities, such as hydroxyl, amino, carboxyl, and sulfhydryl groups, which are suitable for use in attaching a photoreactive group and the aliphatic acid by a suitable chemical reaction, e.g., conventional coupling chemistry.
Alternatively, the spacer can be formed in the course of combining a precursor containing (or capable of attaching) the photoreactive group with another containing (or capable of attaching) the aliphatic acid. For example, the aliphatic acid could be reacted with an aliphatic diamine to give an aliphatic amine derivative of the bifunctional aliphatic acid and which could be coupled with a carboxylic acid containing the photogroup. To those skilled in the art, it would be obvious that the photogroup could be attached to any appropriate thermochemical group which would react with any appropriate nucleophile containing O, N or S.
Examples of suitable spacer groups include, but are not limited to, the groups consisting of substituted or unsubstituted alkylene, oxyalkylene, cycloalkylene, arylene, oxyarylene, or aralkylene group, and having amides, ethers, and carbonates as linking functional groups to the photoactivatible group, and the bifunctional aliphatic fatty acid.
The spacer of the invention can also comprise a polymer which serves as a backbone. The polymer backbone can be either synthetic or naturally occurring, and is preferably a synthetic polymer selected from the group consisting of oligomers, homopolymers, and copolymers resulting from addition or condensation polymerization. Naturally occurring polymers, such as polysaccharides, can be used as well. Preferred backbones are biologically inert, in that they do not provide a biological function that is inconsistent with, or detrimental to, their use in the manner described.
Such polymer backbones can include acrylics such as those polymerized from hydroxyethyl acrylate, hydroxyethyl methacrylate, glyceryl acrylate, glyceryl methacrylate, acrylic acid, methacrylic acid, acrylamide and methacrylamide; vinyls such as polyvinylpyrrolidone and polyvinyl alcohol; nylons such as polycaprolactam; derivatives of polylauryl lactam, polyhexamethylene adipamide and polyhexamethylene dodecanediamide, and polyurethanes; polyethers such as polyethylene oxide, polypropylene oxide, and polybutylene oxide; and biodegradable polymers such as polylactic acid, polyglycolic acid, polydioxanone, polyanhydrides, and polyorthoesters.
The polymeric backbone is chosen to provide a backbone capable of bearing one or more photoreactive groups, and one or more fatty acid functional groups. The polymeric backbone is also selected to provide a spacer between the surface and the various photoreactive groups and fatty acid functional groups. In this manner, the reagent can be bonded to a surface or to an adjacent reagent molecule, to provide the fatty acid functional groups with sufficient freedom of movement to demonstrate optimal activity. The polymer backbones are preferably water soluble, with polyacrylamide and polyvinylpyrrolidone being particularly preferred polymers.
Photoreactive Group
In a preferred embodiment one or more photoreactive groups are provided by the X groups attached to the central Y spacer radical. Upon exposure to a suitable light source, each of the photoreactive groups are subject to activation. The term xe2x80x9cphotoreactive groupxe2x80x9d, as used herein, refers to a chemical group that responds to an applied external energy source in order to undergo active specie generation, resulting in covalent bonding to an adjacent chemical structure (e.g., an aliphatic carbon-hydrogen bond).
Preferred X groups are sufficiently stable to be stored under conditions in which they retain such properties. See, e.g., U.S. Pat. No. 5,002,582, the disclosure of which is incorporated herein by reference. Latent reactive groups can be chosen that are responsive to various portions of the electromagnetic spectrum, with those responsive to ultraviolet and visible portions of the spectrum (referred to herein as xe2x80x9cphotoreactivexe2x80x9d) being particularly preferred.
Photoreactive aryl ketones are preferred, such as acetophenone, benzophenone, anthraquinone, anthrone, and anthrone-like heterocycles (i.e., heterocyclic analogues of anthrone such as those having N, O, or S in the 10- position), or their substituted (e.g., ring substituted) derivatives. The functional groups of such ketones are preferred since they are readily capable of undergoing the activation/inactivation/reactivation cycle described herein. Benzophenone is a particularly preferred photoreactive group, since it is capable of photochemical excitation with the initial formation of an excited singlet state that undergoes intersystem crossing to the triplet state. The excited triplet state can insert into carbon-hydrogen bonds by abstraction of a hydrogen atom (for example, from a support surface or target molecule in the solution and in bonding proximity to the agent), thus creating a radical pair. Subsequent collapse of the radical pair leads to formation of a new carbon-carbon bond. If a reactive bond (e.g., carbon-hydrogen) is not available for bonding, the ultraviolet light-induced excitation of the benzophenone group is reversible and the molecule returns to ground state energy level upon removal of the energy source. Hence, photoreactive aryl ketones are particularly preferred.
The azides constitute a preferred class of latent reactive groups and include arylazides (C6R5N3) such as phenyl azide and particularly 4-fluoro-3-nitrophenyl azide, acyl azides (xe2x80x94COxe2x80x94N3) such as ethyl azidoformate, phenyl azidoformate, sulfonyl azides (xe2x80x94SO2xe2x80x94N3) such as benzenesulfonyl azide, and phosphoryl azides (RO)2PON3 such as diphenyl phosphoryl azide and diethyl phosphoryl azide. Diazo compounds constitute another class of photoreactive groups and include diazoalkanes (xe2x80x94CHN2) such as diazomethane and diphenyldiazomethane, diazoketones (xe2x80x94COxe2x80x94CHN2) such as diazoacetophenone and 1-trifluoromethyl-1-diazo-2-pentanone, diazoacetates (xe2x80x94COxe2x80x94CN2xe2x80x94COxe2x80x94Oxe2x80x94) such as t-butyl alpha diazoacetoacetate. Other photoreactive groups include aliphatic azo compounds such as azobisisobutyronitrile, diazirines (xe2x80x94CHN2) such as 3-trifluoromethyl-3-phenyldiazirine and ketenes (xe2x80x94CHxe2x95x90Cxe2x95x90O) such as ketene and diphenylketene.
Upon activation of the photoreactive groups, the coating adhesion molecules are covalently bound to each other and/or to the material surface by covalent bonds through residues of the photoreactive groups. Exemplary photoreactive groups, and their residues upon activation, are shown as follows.
Preparation of Reagents
Reagents of the present invention can be prepared by any suitable means, depending upon the selection of either a heterobifunctional reagent or a polymeric reagent. In the case of the heterobifunctional reagents, the fatty acid residue is provided by a fatty acid possessing a chemically reactive group on the alkyl chain which permits covalent coupling of the remainder of the heterobifunctional molecule to the fatty acid with preservation of the carboxylic acid functionality. Preferably, the site of the reactive group is in close proximity to the carboxylic acid group so as to minimize effects on the binding activity of the hydrophobic alkyl chain. Most preferably, the fatty acid residue can be provided by a compound such n-tetradecylsuccinic anhydride (TDSA). Reaction of such a molecule with a second molecule possessing a nucleophilic species such as a primary amine results in opening of the anhydride ring to give a fatty acid with an amide linkage to the remainder of the molecule. This reaction generates a pair of regioisomers depending upon the direction of the anhydride ring opening. The second molecule in this reaction can be provided by a spacer group, with or without a photoactivatible group, which possesses a group capable of reaction with the fatty acid compound. Most preferably, this spacer group possesses an amine which is highly reactive with an anhydride species. The spacer group is typically a bifunctional molecule which can have the photoactivatible group attached prior to reaction with the fatty acid derivative or the reverse order of reaction can be used. The bifunctional spacer can be either heterobifunctional or homobifunctional, with the former requiring a differential reactivity in the first and second reaction steps and the latter requiring an efficient method of separating the monofunctionalized spacer following the first reaction. Optionally, no spacer is required and a photoactivatible group possessing functionality capable of reaction with the fatty acid derivative can be used. The above examples are nonlimiting and the methods of accomplishing these coupling reactions are apparent to those skilled in the art.
Polymeric reagents of the invention can be prepared by derivatization of preformed polymers possessing reactive groups along the backbone of the polymer capable of reaction with the photoactivatible groups and the fatty acid derivatives. For example, polyacrylamide, polyvinylpyrrolidone, or siloxanes functionalized with amine groups along the backbone, with or without a spacer group, can be reacted with 4-benzoylbenzoyl chloride (BBA-Cl) and TDSA to provide the photoactivatible and fatty acid ligands respectively. Alternatively, the photoactivatible and fatty acid groups can be prepared in the form of polymerizable monomers which can then be copolymerized with themselves and other monomers to provide polymers of the invention. In a further embodiment of the invention, the photoactivatible group can be introduced in the form of a chain transfer agent along with the fatty acid monomer and other comonomers so as to provide a polymer having the photoactivatible group at the end of the polymer chain. For example, a chain transfer agent possessing two derivatized benzophenones as the photoactivatible groups and a mercaptan as the chain transfer agent can be used to copolymerize a fatty acid monomer and acrylamide or N-vinylpyrrolidone monomers to provide polymers of the invention. Alternatively, this polymer could be prepared with reactive groups along the backbone, followed by reaction with a fatty acid derivative.
Surfaces and Methods of Attachment.
The reagent of the present invention can be used to modify any suitable surface. Where the latent reactive group is a photoreactive group of the preferred type, it is particularly preferred that the surface provide abstractable hydrogen atoms suitable for covalent bonding with the activated group.
Plastics such as polyolefins, polystyrenes, poly(methyl)methacrylates, polyacrylonitriles, poly(vinylacetates), poly (vinyl alcohols), chlorine-containing polymers such as poly(vinyl) chloride, polyoxymethylenes, polycarbonates, polyamides, polyimides, polyurethanes, phenolics, amino-epoxy resins, polyesters, silicones, cellulose-based plastics, and rubber-like plastics can all be used as supports, providing surfaces that can be modified as described herein. See generally, xe2x80x9cPlasticsxe2x80x9d, pp. 462-464, in Concise Encyclopedia of Polymer Science and Engineering, Kroschwitz, ed., John Wiley and Sons, 1990, the disclosure of which is incorporated herein by reference. In addition, supports such as those formed of pyrolytic carbon and silylated surfaces of glass, ceramic, or metal are suitable for surface modification.
Any suitable technique can be used for reagent binding to a surface, and such techniques can be selected and optimized for each material, process, or device. The reagent can be successfully applied to clean material surfaces as listed above by spray, dip, or brush coating of a solution of the fatty acid binding reagent. The surface may be air-dried prior to illumination or the surface can be illuminated while submerged in the coating solution. The photoreactive group is energized via an external stimulation (e.g., exposure to a suitable light source) to form, via free active specie generation, a covalent bond between the reagent and either another polybifunctional reagent molecule or the biomaterial surface. This coating method is herein termed the xe2x80x9cone step coating methodxe2x80x9d, since photoreactive coupling chemistry attaches an invention polymer to a biomaterial surface, and no subsequent steps are required to add the bioactive group. The external stimulation that is employed desirably is electromagnetic radiation, and preferably is radiation in the ultraviolet, visible or infrared regions of the electromagnetic spectrum.
The xe2x80x9ctwo-stepxe2x80x9d method would involve a first step of photocoupling a hydrocarbon backbone to the surface, followed by a second step of attaching (e.g., thermochemically) one or more fatty acid derivatives to the immobilized backbone. For example, this two step approach could involve covalently attaching a photoreactive hydrocarbon backbone containing nucleophiles which could be used to thermochemically couple fatty acid derivatives to the surface, or directly attaching thermochemical groups (e.g. amines) to the surface, followed by thermochemical attachment of one or more fatty acid derivatives.
Alternatively, chemically reactive groups can be introduced on the surface by a variety of non-photochemical methods, followed by chemical coupling of the fatty acid group to the modified surface. For example, amine groups can be introduced on a surface by plasma treatment with a mixture of methane and ammonia and the resulting amines can then be reached with TDSA to chemically couple the fatty acid derivative to the surface through an amide linkage. When desired, other approaches can be used for surface modification using the reagent of the present invention. This approach is particularly useful in those situations in which a support is difficult to modify using conventional chemistry, or for situations that require exceptional durability and stability of the target molecule on the surface.